If you can get tickets directly through the official Colosseum website, book the Underground. I did it years ago, and then most recently at the end of May. It is so much better than it used to be. If I was 7 (or 9) I would think it was pretty cool. If you're more risk tolerant, then book through a third party operator (like the Roman Guy). If they wind up not getting enough tickets (which seems to be happening a lot), they will cancel and either reimburse you or rebook you (but that can be a headache if you've got other things planned and have to work around them). And if you can rebook, it probably won't be for an underground tour. Or, if you're less risk tolerant, then go ahead and book the Arena Floor tour--I haven't heard of anyone having that cancelled recently (though presumably it could happen). It's also pretty cool. I had a ridiculous experience this last trip--my original Underground tour with Walks of Italy was cancelled four days before (two days before I flew to Italy), I rebooked a "regular" tour with the Roman Guy, they cancelled the morning of, but offered me an Underground tour at a different time that same day. I had to cancel my catacombs tour (also with them) but it all worked out. (Note: The Roman Guy guide was excellent.) Ultimately, I wouldn't say one tour is "top of the line" and the others aren't. It's certainly marketed that way, but its more about the "exclusivity" and doesn't reflect the quality (exclusive doesn't mean better in my book). And as another poster noted, you can definitely still see the Underground from the Arena floor.
